Not yet all the obstacles are defeated:
$ sudo ./flash.sh
Flash utility v1.2
Detected Linux
Searching device to flash..
Found XQ-BT52, serial:HQ61A914E5, baseband:62.0.A.3.109, bootloader:1322-1665_X_Boot_SM6350_LA2.0_R_49
Found matching device with serial HQ61A914E5
Fastboot command: fastboot -s HQ61A914E5
>> fastboot -s HQ61A914E5 getvar secure
<< getvar:secure FAILED (remote: unknown command)
Then I ran the command again:
$ sudo ./flash.sh
Flash utility v1.2
Detected Linux
Searching device to flash..
Found FAILED, serial:HQ61A914E5, baseband:FAILED, bootloader:FAILED
No valid devices found.
Checksums are valid:
$ md5sum -c md5.lst
AdbWinApi.dll: OK
AdbWinUsbApi.dll: OK
dtbo.img: OK
fastboot.exe: OK
flash-config.sh: OK
flash-on-windows.bat: OK
flash.sh: OK
flashing-README.txt: OK
hw-release: OK
hybris-boot.img: OK
hybris-recovery.img: OK
os-release: OK
sailfish.img001: OK
EDIT: Now it succeeded!
I first connected the phone pressing vol up, ran the flash, got Searching device to flash.. then I connected the phone again while pressing vol down and now it ran the flashing successfully.